COPEMYS AND THE BARSTOVIAN HEMINGFORDIAN BOUNDARY/

Boundaries of land mammal ages have rarely been addressed. Now that No rth American land mammal ages are well ordered and reasonably well cal ibrated, it is fitting to seek more rigorous guidelines for the defini tion and recognition of their boundaries. Criteria for recognizing bou ndaries of land mammal ages are discussed briefly, with selected examp les of problems in boundary recognition. The boundary of a land mammal age should be defined as (a) the appearance of a single taxon, prefer ably an immigrant, that is morphologically distinctive, widespread and well represented, taxonomically stable; and (b) strata where the boun dary event is recorded should be well sampled, both above and below th e level where the ''boundary event'' taxon appears. The Hemingfordian/ Barstovian boundary is reviewed and the boundary redefined, based on t he appearance of the cricetid rodent Copemys. This small mammal best m eets the criteria for a ''boundary event'' taxon discussed in the text . The revised Hemingfordian/Barstovian boundary is recognized low in t he Rak Division of the Barstow stratigraphic sequence in the Mud Hills of southern California. This is lower than previous designated bounda ries and equates (more or less) to the lower part of magnetic chron C5 CN, or approximately 16.8 Ma, about 0.8 Ma older than previous calibra tions.